#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<ll>
#define vvi vector<vector<ll>>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<pair<ll, ll>>
#define pp pair<ll, ll>
#define qi queue<ll>
#define qp queue<pp>
#define pqi priority_queue<ll>
#define pqp priority_queue<pp>
#define mi map<ll, ll>
#define mpi map<pp, ll>
#define mip map<ll, pp>
#define mpp map<pp, pp>
#define mb map<ll, bool>
#define si set<ll>
#define sp set<pp>
#define mod 1000000007
#define rep(a, b) for(int a = 0; a < (b); a++)
#define rep2(a, b) for(int a = 1; a < (b); a++)
#define inf 100000000000000
vi A, B;
ll n, m;
bool check(ll x) {
ll ans = 0;
rep(i, n) {
ll c = max(A[i], B[i]);
if (x < m * c) {
ans += (x + c - 1) / c;
}
else {
ans += m;
ll y = x - c * m;
ans += (y + B[i] - 1) / B[i];
}
if (ans > n * m) {
break;
}
}
if (ans <= n * m) {
return true;
}
else {
return false;
}
}
int main()
{
cin >> n >> m;
A.resize(n);
B.resize(n);
rep(i, n) {
cin >> A[i];
}
rep(i, n) {
cin >> B[i];
}
ll MAX = 1000000000000000009;
ll MIN = 0;
while (MIN + 1 < MAX) {
ll s = (MIN + MAX) / 2;
bool bs = check(s);
if (bs) {
MIN = s;
}
else {
MAX = s;
}
}
cout << MIN << endl;
}
